1. Field
Exemplary embodiments of the present invention relate to a delay circuit.
2. Description of the Related Art
Circuits included in an integrated circuit do not operate alone but operate along with other peripheral circuits by exchanging signals, such as data with the peripheral circuits. In general, to make an A circuit and a B circuit perform an interactive operation with each other, the A circuit requests the B circuit for a certain operation, and the A circuit waits until the B circuit performs the operation requested by the A circuit. This waiting time is referred to as latency. Latency is used to increase the transmission efficiency of signals exchanged between the A circuit and the B circuit and to allow for the internal operation time for the B circuit.
For example, an integrated circuit may include a memory controller and a memory device. When a memory controller applies a write command to the memory device, the memory device stores data inputted from the memory controller in a memory cell. The memory device, however, cannot receive the data from the memory controller as soon as it receives the write command because the memory device needs some time to internally prepare for the data storing operation. The writing preparation time is defined as write latency.
Generally, the address for storing data in a memory cell is transferred along with a write command. However, since the data is inputted into the memory device after the write latency time elapses, the address may be delayed by the write latency.
To increase the efficiency of a data bus in Double Data Rate (DDR) 2 Synchronous Dynamic Random Access Memory (SDRAM) devices and DDR3 SDRAM devices, Additive Latency (AL) may be implemented. Even though a read/write command or an address is transferred from the memory controller before a RAS to CAS Delay (tRCD), the command or address is delayed by the additive latency, and an internal read/write command or an internal address is generated after the tRCD, and a read/write operation of the semiconductor memory device is performed after the tRCD passes. The additive latency is set by EMRS (Extended Mode Register Set). The additive latency is set to a particular value in a DDR2 SDRAM device, but the additive latency is interlocked with CAS latency (CL) to 0, CL-1, and CL-2 in a DDR3 SDRAM device. The additive latency is applied to both read commands and write commands and accordingly, the address should be delayed by the additive latency.
Hereafter, a latency control circuit of a memory device that delays a read/write command and an address by an additive latency is described.
FIG. 1 is a block view of a latency control circuit of a conventional memory device.
Referring to FIG. 1, the latency control circuit of the conventional memory device includes a read command delay unit 110, a write command delay unit 120, and address delay blocks 130, 140 and 150. The first to third address delay blocks 130, 140 and 150 includes a delay control unit 130, a first unit bit delay unit 140, and a second unit bit delay unit 150. Here, it is assumed in FIG. 1 that the address is two bits.
Hereafter, the operation of the latency control circuit of the conventional memory device is described with reference to FIG. 1.
As described above, an additive latency either has a particular value or is determined by being interlocked with CAS latency. In the drawing, however, where the additive latency ranges from 0 to 4 clock periods is illustrated.
The read command delay unit 110 includes a plurality of D-flipflops 111 to 114 and a first selector 115. The D-flipflops 111 to 114 delays a signal inputted into the D-flipflops by one clock period and outputs a delayed signal in synchronization with a clock CLK. Therefore, when a read command RDCMD of 111 is inputted, the output signals OUT1A to OUT4A of the multiple D-flipflops 111 to 114 are sequentially output at an interval of one clock period.
The first selector 115 selects one signal among the read command RDCMD and the output signals OUT1A to OUT4A of the multiple D-flipflops 111 to 114 to be a delayed read command RDCMD_DEL. The first selector 115 selects one signal in response to latency information LATENCY. For example, when the additive latency is 0, the first selector 115 selects the read command RDCMD as a delayed read command RDCMD_DEL, and when the additive latency is 2, the first selector 115 selects the output signal OUT2A of the D-flipflop 112 as a delayed read command RDCMD_DEL.
In FIG. 1, the latency information is a single-bit or multi-bit digital signal whose bit number is changed according to the length of the additive latency.
The write command delay unit 120 includes a plurality of D-flipflops 121 to 124 and a second selector 125. The second selector 125 selects one signal among the signals including a write command WTCMD and the output signals OUT1B to OUT4B of the multiple D-flipflops 121 to 124 to be a delayed write command WTCMD_DEL. The second selector 125 selects one signal in response to latency information LATENCY. The structure and operation of the write command delay unit 120 is the same as the read command delay unit 110.
The delay control unit 130 of the address delay block 130, 140 and 150 includes a plurality of D-flipflops 131 to 134. The D-flipflops 131 to 134 delays a signal inputted thereto by one clock period and outputs a delayed signal in synchronization with a clock CLK. When a read command RDCMD or a write command WTCMD is applied to the memory device, an enabled command signal CMDS is inputted. Therefore, the outputs OUT1C to OUT4C of the multiple D-flipflops 131 to 134 are enabled at an interval of one clock period after the read command RDCMD or the write command WTCMD is applied.
The first unit bit delay unit 140 includes a plurality of latches 141 to 144 and a third selector 145. The first unit bit delay unit 140 delays a first bit ADD<0> of a 2-bit address that is applied to the first unit bit delay unit 140. The latches 141 to 144 respectively correspond to the D-flipflops 131 to 134. When the output of a D-flipflop corresponding thereto is enabled, the latch stores and outputs a signal OUT1D to OUT4D inputted thereto. The third selector 145 selects one among the first bit ADD<0> of the 2-bit address and the outputs OUT1D to OUT4D of the multiple latches 141 to 144 as a first bit ADD<0>_DEL of a delayed address. In this way, the first bit ADD<0> of the 2-bit address may be delayed by a command signal CMDS is delayed. In short, the first bit ADD<0> of the 2-bit address may be delayed equal to the read command RDCMD or the write command WTCMD is delayed.
The second unit bit delay unit 150 generates a second bit ADD<1>_DEL of the delayed address by delaying a second bit ADD<1> of the received 2-bit address ADD<0:1>. The structure and operation of the second unit bit delay unit 150 are the same as those of the first unit bit delay unit 140.
FIG. 2 is a block view of a D-flipflop and a latch shown in FIG. 1.
The D-flipflop generates an output signal O by synchronizing an input signal I with a clock C, or when the input signal I is synchronized with the clock C, the D-flipflop generates the output signal O by delaying the input signal I by one clock period. When a reset signal RST is enabled, the output signal O is initialized. Since the reset signal RST is used only when the D-flipflop is initialized, an end to which the reset signal RST is applied is not illustrated in the D-flipflop block diagram shown in FIG. 1, and it is not illustrated in any subsequent figures either. More specifically, the input signal I passes through a first pass gate 201 at a falling edge of the clock C to be stored in a first storage 202. The value stored in the first storage 202 passes through a second pass gate 203 at a rising edge of the clock C to be stored in a second storage 204. The stored values are transferred as the output signals O.
The latch stores the input signal I and outputs it as the output signal O when a pass signal S is enabled. More specifically, the input signal I passes through an inverter 205 and is stored in a storage 206 when the pass signal S is enabled. The input signal I transferred as the output signal O.
Here, the number of logic gates included in the D-flipflop is much more than the number of logic gates included in the latch. Therefore, the D-flipflop has a wider circuit area than the latch. Also, a D-flipflop may consume more current than a latch. However, while the D-flipflop may delay the input signal I by one clock period, the latch may delay the input signal I by one clock period only when it uses the output signal O of the D-flipflop as the pass signal S.
A latency control circuit of a conventional memory device includes the read command delay unit 110, the write command delay unit 120, and the delay control unit 130 and each of the read command delay unit 110, the write command delay unit 120, and the delay control unit 130 include a plurality of D-flipflops. Therefore, the area and current consumption of the latency control circuit of the memory device are great. A delay circuit for delaying two different signals includes two delay units that respectively delay the signals, and each of the delay units includes a plurality of D-flipflops. Therefore, the same drawback as the latency control circuit of the conventional memory device appears.